Re: P.S. to John!!!

[P3D John W Roberts <roberts@xxxxxxxxxxxxxxxxx>]

>Date: Tue, 15 Oct 1996 19:21:21 -0500
>From: P3D Allan Woods <allanwx@xxxxxxxxxx>
>Subject: P.S. to John!!!

>>Maybe Bill's system uses "circle of confusion"
>>as the encoding path but people don't.

>Ah, this is fodder for another one of those never-ending opinion-fests...
>but, I believe they DO! (use "circle of confusion").

I agree - I made some mention of it in the list of ways people perceive
depth (first posted in 1985, I think).

>Using just one eye, you know what is closer or farther away because,
>among other things, you need to re-focus when you look elsewhere along
>the Z-axis.

>Part of the subtle beauty of the human mechanism is that we "know" not only
>that we are re-focusing, but whether we are focusing closer or farther.

Also you can get some idea of distances of objects even without focusing on
them by the amount of blur (absolute or relative to other objects). Of course
for a given focal distance and a given amount of blur there are two solutions
(in front of and beyond the focal distance), so additional information is
needed to resolve the issue.

>This re-focusing is triggered by the existence of the "circle of
>confusion" if we look at something which is out of focus.

Since depth of field is dependent partly on the degree of dilation of the
pupil, the "scale" will vary based on the amount of lighting. It would be
interesting to find out whether people's depth judgement (one eye, head
motionless) is better in dimmer light than in brighter light, and whether in
estimating an absolute distance to an object they properly compensate for
effects related to the degree of pupil dilation. (And also whether there are
different results if they've just had an eye exam and been given those eye
drops that force the pupils to dilate.)

The mechanism you described may be less affected on an absolute scale by
differences in dilation than the additional method I described, but even in
that case a smaller pupil and greater depth of field gives less information to
work with (the "cone" gets skinnier, so the distance to the apex is harder to
pinpoint).

John R


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